A silicon charge-coupled device (CCD) imager is preferably used at a temperature in the range from about -100 degrees C. to -150 degrees C. in order to reduce the dark current to an acceptable level. It is conventional to achieve this low temperature by mounting the imager on a copper block that is in heat exchange relationship with a vessel that contains a liquid cryogen, for example liquid nitrogen (LN.sub.2). At standard pressure, liquid nitrogen evaporates at approximately -196 degrees C. It is not desirable to operate the imager at a temperature as low as -196 degrees C. because carrier freeze-out typically occurs, rendering the device inoperative. In order to elevate the temperature of the imager to the desired operating range, the thermally-conductive coupling between the imager and the liquid cryogen vessel is kept quite loose, e.g. by keeping the copper block physically spaced from the liquid cryogen vessel and connecting the copper block to the vessel using copper braids. Moreover, a temperature sensor is mounted on the copper block and a heat source, such as a resistive heater, is coupled to the thermal path between the copper block and the liquid cryogen vessel. A control circuit controls the supply of current to the resistive heater, in dependence upon the temperature sensed by the sensor, and therefore the temperature of the copper block and of the imager carried thereby is controlled with a degree of accuracy that is generally satisfactory.
However, the conventional technique is far from ideal, and in particular is subject to the disadvantages that the heat sensor is spaced from the imager and therefore does not sense the temperature of the imager, and the spacing of the heater from the sensor implies that there is a considerable time delay between a change in the temperature sensed by the sensor and the corresponding countervailing change in temperature caused by corrective action of the control circuit.